The present invention relates to illumination devices, and more particularly to a light emitting apparatus having a plurality of light emitting diodes for illuminating a region such as an instrument panel.
In electronic measurement instruments, such as oscilloscopes, television waveform monitors, vectorscopes and the like, a waveform and/or a vector of an input signal is displayed on a screen of a cathode ray tube (CRT). Many kinds of graticules or scales are provided on the inside or outside of the CRT faceplate in order to assist an operator in the observation of the displayed waveform or vector. Such a graticule pattern may be a grid pattern or any desired pattern representative of an allowable range for the waveform or the vector. An illumination device is used to light the graticule pattern so that the operator may measure the input signal with the graticule in a darkened room. Such illumination devices are used in other fields, for example, as brake indicating lamps for automobiles.
In illumination devices for a CRT graticule or a brake lamp, high intensity is desired and there should provision for malfunction of the devices. A conventional illuminating device uses an incandescent lamp and a colored glass or a semitransparent colored plastic to emit the colored light therefrom. Since the lifetime of an incandescent lamp is limited, there is a high possibility that the filament of the lamp will open suddenly. It is troublesome to replace the broken lamp with a new lamp every time the lamp breaks. In brake lamps for an automobile, it may be dangerous if the lamp breaks down suddenly.
Recently light emitting diodes (LEDs) have been used instead of an incandescent lamp because LED lifetime is longer than incandescent lamps and LED intensity levels have been improved. Therefore LEDs may be used to overcome the above discussed disadvantages. However the intensity of a single LED is not enough to illuminate the CRT graticule or indicate brake condition. A plurality of LEDs are necessary to equal the intensity level of a single incandescent lamp.
When LEDs are used for illumination devices, a current source applies a current to each LED. For a simple circuit configuration, all the LEDs are connected in series and this series circuit receives the current from a single current source. As described above the lifetime of an LED is longer than that of an incandescent lamp. However LED lifetime is not infinite and it may break down under many different conditions. When at least one LED breaks down in the series circuit, all the LEDs stop emitting light. Thus, this does not satisfy maintenance and safety requirements.
If all the LEDs are connected in parallel and receive the current from a single current source, the luminance levels of the LEDs are different from each other because of the different characteristics of the LEDs which cause the currents flowing through the LEDs to be different from each other. Thus, this configuration does not emit a uniform light. In addition, if any one of the LEDs becomes shorted, the current from the current source flows through only the shorted LED and the other LEDs do not emit light. However if each of the LEDs has its own current source, the circuit configuration becomes expensive and complex.
Additionally an LED is sensitive to temperature and LED lifetime may be reduced if a high current flows through it at a high temperature. Therefore the current flowing through the LED should be within a forward current derating curve of the LED, i.e., a characteristic curve of the available current at each temperature for the LED.
What is desired is a light emitting apparatus that includes a plurality of LEDs and reduces the number of current sources configured so that a broken LED and/or current source does not affect other LEDs and/or current sources. Also adjustment of the current flowing through the LEDs as a function of temperature is desired to prolong the LED lifetime.